Photosynthesis and photoprotection in top leaves respond faster to irradiance fluctuations than bottom leaves in a tomato canopy

Accounting for the dynamic responses of photosynthesis and photoprotection to naturally fluctuating irradiance can improve predictions of plant performance in the field, but the variation of these dynamics within crop canopies is poorly understood. We conducted a detailed study of dynamic and steady-state photosynthesis, photoprotection, leaf pigmentation, and stomatal anatomy in four leaf layers (100, 150, 200 and 250 cm from the floor) of a fully-grown tomato canopy in the greenhouse. We found that leaves at the top of the canopy exhibited higher photosynthetic capacity and faster photosynthetic induction (shorter time to reach 50% of full photosynthetic induction; t 50 -A ) compared to lower-canopy leaves, accompanied by higher stomatal conductance and a faster activation of carboxylation and linear electron transport capacities. In upper-canopy leaves, non-photochemical quenching showed faster induction and relaxation after in- and decreases in irradiance, allowing for more effective photoprotection in these leaves. A large number of leaf functional traits, including photosynthetic capacity, chlorophyll a : b ratio, stomatal density and specific leaf area, correlated strongly with the irradiance integral and the red:far-red ratio, both of which showed strong gradients throughout the canopy. The rate of stomatal movement after in- or decreases in irradiance was not strongly affected by leaf layer, and neither was the rate of loss of photosynthetic induction under low irradiance. Time-averaged photosynthesis under fluctuating irradiance was strongly correlated to steady-state photosynthesis under high irradiance, and to a lesser extent to t 50 -A , suggesting that both steady-state photosynthetic capacity and the rapidity of photosynthesis response to a change in irradiance affect time-integrated dynamic photosynthesis.